The present invention relates to a method of manufacturing prismatic batteries, and more particularly to an improved method for hermetically sealing a prismatic case housing power generating elements by welding a sealing plate on the open end of the prismatic case.
By way of example, secondary batteries used as power sources for portable electronic instruments are required to be of a high energy density and, at the same time, are required to be of a shape efficient in terms of space used in order to achieve weight-lightness and compactness. A lithium ion secondary battery which uses a prismatic aluminum casing is a battery which fulfills these requirements.
Because the lithium ion secondary battery, in terms of its structure apart from anything else, is required to be hermetically stable over a long time, a sealing plate is bonded by laser welding to the open end of a prismatic case formed as a square tube shape with a bottom, thereby sealing its open end. Such a laser welding method has a characteristic of excellent operation efficiency as compared with other welding methods, since it causes less thermal effect on the electrolyte and electrically insulated portions housed within the casing.
Conventional methods for manufacturing prismatic batteries by laser welding of the prismatic case and sealing plate include those disclosed in Japanese Patent Laid-open No. 8-315788, Japanese Patent Laid-open No. 8-315789, and Japanese Patent Laid-open No. 8-315790. In these disclosed manufacturing methods, the sealing plate is fitted into the open end of a prismatic case, which is positioned with the open end facing upwards. The laser beam is irradiated from a vertical direction toward the abutment area of the prismatic case and sealing plate; and this abutment area is scanned with the laser beam and welded, whereby the open end of the prismatic case is hermetically sealed with the sealing plate.
However, in order to weld the abutment area between the sealing plate and the prismatic case which is disposed upright with its open end facing upwards with the laser beam that is irradiated from the vertical direction, it is necessary to move either the laser beam or the prismatic case along the welding line formed of straight lines of four sides linked by each corner at the open end of the case, wherefore control of scanning the abutment line to be welded with the laser beam becomes complex, thus causing a problem of low production efficiency. Also, since weld penetration caused by welding occurs towards the inside of the battery as the abutment area is welded by the laser beam irradiated from the vertical direction, there is a risk of molten metal infiltrating the battery if the amount of penetration is increased. Moreover, the precision to which the prismatic case and sealing plate can be finished does have limits. Even if the amount of penetration during welding is optimized so as not to reach into the battery, penetration may reach into the battery due to variations in the finishing precision of the thin materials. In such a case, molten metal dropping into the battery may cause internal short circuits. Moreover, although laser welding is a welding method which does not have much thermal influence on the electrolytic solution and electrical insulating matter, since the heating is directed towards the battery interior, thermal effects are difficult to eliminate because of limits to welding precision or to the finishing precision of the prismatic case and sealing plate.